1. Field of the Invention
The present invention relates to an electrochromic display device and, more particularly, to an electrochromic display device which utilizes a solid proton conductive material for an electrolyte and which is formed by printing or application method.
2. Description of the Prior Art
With recent advances in electronics, electrochromic display devices (to be referred to as an ECD for brevity hereinafter) are being studied as digital display elements which operate electrically. An ECD is defined as a display device which utilizes the particular behaviour of some materials according to which the light absorption changes by application of a voltage. Many such materials are known including organic and inorganic materials. However, examples of electrochromic materials (to be referred to as an EC for brevity hereinafter which may be used in consideration of display reliability include organic materials such as viologen derivatives and inorganic materials such as transition metal oxides particularly tungsten oxide and molybdenum oxide. ECDs utilizing liquid electrolyte for the display cells are superior in electrochromic characteristics and are almost in the stage of practical use. However, since they utilize liquid electrolyte, high sealing technics is required, resulting in high cost. For an ECD utilizing a transition metal oxide for an EC layer, it has been proposed to make the overall ECDs solid by combination with an insulation film which is formed by deposition to a thin film of SiO.sub.2, Cr.sub.2 O.sub.3, CaF.sub.2, MgF.sub.2, ZnO.sub.2, Na-.beta.-Al.sub.2 O.sub.3 or the like. However, such an ECD presents the problems of a low contrast and a relatively slow response. For example, U.S. Pat. Ser. No. 3,521,941 teaches the use of an insulator such as SiO, CaF.sub.2, MgF.sub.2 or the like for a current carrier permeable insulator such as proton (H.sup.+). However, the proposed device has a response time of 30 seconds at 5 to 8 V, which hardly allows practical application. Additionally, the insulation film used in this patent must be made as thin as 0.001 to 1.0 .mu.m. The method for forming such a thin film is limited to the deposition method, and control of the film thickness is difficult.
The coloration of an amorphous tungsten deposited film (WO.sub.3 film) when it is used as the EC layer is explained, by Faughnan (Appl. Phys. Lett 26 120 1975) in terms of the double injection of proton and electron as shown below: EQU XH.sup.+ +WO.sub.3 +Xe.sup.- .revreaction.Hx.sup.+ WO.sub.3 .multidot.e.sup.x-
ECDs are conventionally known which use display elements of transition metal oxides such as tungsten oxide as the electrochromic material and which combines this display element with an electrolyte having an equivalent concentration of 0.1 such as sulfuric acid. With this ECD, the tungsten oxide EC layer is colored blue upon application of a voltage. For example, as shown in FIG. 1, a thin layer of a transparent electrode 2 is formed over a transparent substrate 1 of a material such as glass in a pattern of display. An EC layer 3 of tungsten oxide or viologen derivative as described above is formed thereover. A counter electrode 6 is arranged in opposition to the laminate body obtained through a light dispersing plate 5. A sealing cover 7 is formed so as to seal the display part in an electrolyte 4, that is, liquid electrolyte.
However, since the liquid electrolyte is used, the cell structure becomes complex owing to the necessity of preventing leakage of the liquid electrolyte. Furthermore, since the EC layer of tungsten oxide or the like dissolves in liquid electrolyte such as sulfuric acid, it is desired to use a solid electrolyte. In consideration of this, Japanese Laid-Open Pat. application No. 12,348/78 proposes an ECD which uses a proton conductive layer of an acid addition salt consisting of hexamethylenetetramine and sulfuric acid. Since this proton conductive layer is in the form of white crystals, a clear display of blue on a white background is obtained when this proton conductive layer is used for the ECD. With this ECD, however, the proton conductivity is low. If the proton conductive layer is made thin in order to obtain faster response, the color of the counter electrode becomes slightly visible. Then, the color density of the background increases, so that the proton conductive layer must be made whiter. In order to achieve clear display, it is conventionally proposed to use a white pigment. For example, Japanese Laid-Open Pat. application No. 99,057/76 proposes addition of titanium oxide (TiO.sub.2) to the electrolyte. Titanium oxide is an insulator and has a resistivity of 1.times.10.sup.8 .OMEGA..multidot.cm. If the resistance is low as in the case of a liquid electrolyte, the addition of titanium oxide does not degrade the response time very much. However, the proton conductive layer consisting of hexamethylenetetramine and sulfuric acid has a low proton conductivity. Thus, the addition of titanium oxide degrades the response time although it improves the clearness of the display.
The above is the state of the field. However, the fact that the electrolyte layer may be formed by a simple method if the cell is totally made of a solid material by use of a proton conductive material as an electrolyte for the ECD still deserves some consideration.
When such an ECD is compared with the liquid ECD, the cell structure becomes simpler and the gap between the electrodes need not be controlled as strictly as for the liquid crystal cells. Therefore, a solid ECD is particularly effective for displaying in a larger area. The problem is too long a response time of the display device due to the low conductivity of the conventional solid electrolyte. A solid electrode of high conductivity has thus been desired.